1. Field of the Invention
The present invention relates to the fabrication of an MR read head having a laminated magnetic shield structure for improved head stability and performance.
2. Description of the Related Art
A magneto-resistive (MR) read head consists of an MR sensor element that is positioned between two magnetic shields and electrically insulated from them by thin dielectric layers. The read head is typically a part of a combined read/write head assembly which also contains a magnetic inductive write head portion. The purpose of the magnetic shields is to exclude stray magnetic fields from the sensor element. Stray fields, in this context, include such fields as are produced by magnetically encoded data on portions of the magnetic medium not being accessed by the read/write head, as well as fields produced by the write head portion of the read/write assembly during its normal operation.
The operation of magnetic shields can be understood by referring to a simple schematic diagram of an MR sensor and one of its shields as is depicted in FIG. 1. The MR sensor is a device consisting of a lamination of magnetic and non-magnetic layers whose resistance is a function of the angle between the magnetization of the magnetic layers and the direction of current flow through them. A constant flow of current maintained through the sensor then allows variations of its magnetization direction to be sensed as voltage changes, which thereby constitute the readback signal. The sensor is fabricated as part of a combined read/write head assembly which is fabricated as part of a device called a slider and, in turn, is mounted on an actuator assembly. As a magnetic storage medium, such as a hard disk, moves rapidly beneath it, the slider literally flies on a layer of air and is positioned by the actuator at the proper position for reading or writing data on the magnetic medium. The relative motion of medium and slider produces rapidly varying magnetic fields at the sensor. These fields cause the magetization of the sensor to change. Recent types of MR sensors, such as spin valve giant magneto-resistive (SVGMR) sensors, are extremely sensitive to minute variations of a single magnetized layer (the ferromagnetic free layer) of the sensor lamination. As technological advances continue to increase the area density of encoded data on the magnetic medium, the MR sensor must be capable of discriminating between extremely small regions of encoded data, those that it is accessing and those that surround it. It is the task of the magnetic shields to aid in this discriminatory process by eliminating the effects of extraneous magnetic fields to as great a degree as possible, to do so in a reproducible fashion and to further do so without introducing noise into the system. Furthermore, as area densities of encoded data increase, the read gap of the read head must be made correspondingly thinner, which decreases the distance between the MR sensor element and its surrounding shield. This, in turn, increases the interaction between the magnetic properties of the shield and those of the sensor element, exacerbating whatever problems may exist in the shield design.
Magnetic shields fabricated in accordance with the prior art are single layers of magnetic material such as Sendust (FeSiAl). These layers are not magnetized in a substantially single direction (uniaxial anisotropy), but are partitioned into a relatively isotropic pattern of domains, each of which is characterized by it own magnetization direction and bounded by a domain wall structure. When the shield layer is in thermal and mechanical equilibrium, the domains arrange themselves in a pattern of lowest total magnetic energy. The particular domain pattern established in a given shield layer under normal operating conditions depends on many factors, some of which are fixed and some of which are variable. Fixed factors include layer shape, layer dimensions, material composition of the layer and defect structure within the layer. Variable factors include the thermal condition of the layer and its state of mechanical stress resulting from that thermal condition or from factors such as the motion of the head over the rapidly moving disk surface or magnetostriction. Ideally, the shield operates by changing the direction of its magnetization vectors in response to the influence of external magnetic fields. These vectors should return to their original states when the read head is quiescent. Unfortunately, the domain walls often split apart and merge as a result of stress variations. When this occurs, the walls may try to return to their original pattern, but will do so on a time scale that is long compared to the field variations that are being read by the MR sensor. As a result, the sensor is operating in an environment in which the magnetic structure of its shields is changing. The changing domain structure of the shields as well as the variations in its magnetization produced by the fields in its environment induces eddy currents within those shields, which can itself generate further local stresses. In addition, the changing domain structure induces a noisy signal in the MR sensor, producing what is called Barkhausen noise in the sensor's readout signal. A final, but by no means less significant disadvantage of the relatively isotropic arrangement of magnetizations of a typical shield, is that it makes it difficult to maintain the fixed bias of the magnetization of the MR sensor element itself. In order for the sensor element to operate in an optimal portion of its response curve (i.e. response of its resistance to the external magnetic field), the sensor's magnetization is “biased,” or fixed in a direction in the plane of its air-bearing surface. This biasing is typically accomplished by magnetic layers that are formed integrally with the sensor element during its fabrication. It would be of great importance that the dominant magnetization of the shield be in the same direction as the desired bias direction of the sensor.
It is clear, therefore, that the performance of magnetic shields can be substantially improved if their domain structure can be stabilized and their magnetization made maximally uniaxial. One way to do this, is by “pinning” the magnetization of the shield material so that it assumes a fixed direction (uniaxial anisotropy) to which it returns during the operation of the read/write head assembly. The invention of Gill et al. (U.S. Pat. No. 5,621,592) teaches a method of forming a magnetic shield with substantial uniaxial anisotropy by laminating a ferromagnetic layer of NiFe with an antiferromagnetic layer of NiMn. The antiferromagnetic layer pins the magnetization of the ferromagnetic layer by exchange coupling, so that the magnetization of the ferromagnetic layer has a very strong tendency to rotate back to its easy axis after responding to fields that rotate it away from that axis.
In a somewhat different vein, the invention of Ravipati (U.S. Pat. No. 5,838,521) teaches a method of forming a laminated shield in a tri-layer configuration, in which two layers of ferromagnetic material such as NiFe are separated by an equally thick layer of non-magnetic material, such as tantalum. The two ferromagnetic layers are formed with their uniaxial magnetizations parallel, but oppositely directed. Particular advantages of this shield configuration are: 1) that there is a single domain that is substantially coextensive with the planar surface of the shield and, 2) magnetization vectors of the respective layers, being oppositely directed, form closed paths encompassing both layers. This closure tends to reduce the occurrence of eddy currents in the shield.
It has been relatively recently discovered that magnetic multilayers, in which layers of ferromagnetic material are separated by very thin layers of non-magnetic material, exhibit an unusual form of oscillatory exchange coupling that depends critically on the thickness of the non-magnetic layer. As the thickness of the non-magnetic layer is varied, the two ferromagnetic layers will change from an antiferromagnetic coupling, in which their magnitizations are pinned in opposite directions, to a ferromagnetic coupling, in which their magnetizations are parallel and in the same direction. The explanation for this phenomenon is that the non-magnetic layer forms a trapping channel for conduction electrons due to spin dependent scattering from its interfaces with the ferromagnetic material. The trapping channel thereby serves as a quantum well whose spin-polarized energy levels are a function of the width of the non-magnetic layer. As the thickness of the non-magnetic layer is increased, the energy levels shift until the Fermi level is crossed, at which point the interaction between the two ferromagnetic layers will change from antiferromagnetic to ferromagnetic. This form of quantum mechanical exchange coupling provides an extremely effective mechanism for the formation of a magnetic shield of great stability. It is the object of the present invention to teach a method of forming a laminated tri-layer shield that is pinned in an antiferromagnetic configuration by means of the quantum mechanical oscillatory exchange coupling discussed above.